Owing to progress in digital information communication networks and others such as the Internet in recent years, users can now easily access network information through individual-oriented terminals using cellular phones or the like.
In such a digital information communication network, information is transmitted by digital signals. Even when an individual user copies music or video data transmitted via the aforementioned information communication network, it is now possible to obtain copied data without degrading audio and/or image qualities.
Therefore, the copyright of the owner may be significantly infringed unless appropriate measures are taken for copyright protection when a copyrighted content such as music data or image data is transmitted over the digital information communication network.
However, if copyright protection is given top priority, it may become impossible to distribute content data over the fast-growing digital information communication network. This impairs an interest of the copyright owner, who can essentially collect predetermined copyright royalties for copies of the copyrighted data.
Instead of the distribution over the digital information communication network described above, distribution may be performed via record mediums storing digital data. In connection with the latter case, music data recorded on CDs (compact disks) on the market can be freely copied in principle onto magneto-optical disks (e.g., MDs) as long as the copied music is only for the personal use. However, personal users performing digital recording or the like indirectly pay predetermined amounts in prices of digital recording devices and mediums as guaranty moneys to a copyright owner.
In view of the fact that the music data is digital data, which does not cause copy deterioration of information when it is copied as digital signals from a CD to an MD, such devices and structures are employed for copyright protection that the copied music information cannot be copied as digital data from the recordable MD to another MD.
In view of the above, the public distribution itself of the content data such as music data and image data over the digital information communication network is restricted by the public transmission right of the copyright owner, and therefore sufficient measures must be taken for the copyright protection in such distribution.
In the above case, it is necessary to inhibit unauthorized further copying of the content data such as copyrighted music data or image data, which was once sent to the public over the digital information communication network.
Such a data distribution system has been proposed that a distribution server holding the encrypted content data distributes the encrypted content data to memory cards attached to terminal devices such as cellular phones via the terminal devices. In this data distribution system, a public encryption key of the memory card, which has been certified by a certification authority, and its certificate are sent to the distribution server when requesting the distribution of the encrypted content data. After the distribution server confirms the reception of the certified certificate, the encrypted content data and a license key for decrypting the encrypted content data are sent to the memory card. When distributing the license key, the distribution server and the memory card generate session keys, which are different from those generated in other distribution. With the session keys thus generated, the public encryption key is encrypted, and the keys are exchanged between the distribution server and the memory card.
Finally, the distribution server sends the license, which is encrypted with the public encryption key peculiar to each memory card, and is further encrypted with the session key, as well as the encrypted content data to the memory card. The memory card records the license key and the encrypted content data thus received in the memory card.
When the encrypted content data recorded in the memory card is to be reproduced, the memory card is attached to the cellular phone. In addition to an ordinary function of the telephone, the cellular phone has a dedicated circuit for decrypting the encrypted content data sent from the memory card, and reproducing it for external output.
As described above, the user of the cellular phone can receive the encrypted content data from the distribution server via the cellular phone, and can reproduce the encrypted content data.
In another manner, encrypted content data is distributed over the Internet to personal computers. For distributing the encrypted content data to the personal computers, software installed in the personal computer is used for distributing the encrypted content data. Security for the encrypted content data in a distribution using the software is lower than that achieved by writing the encrypted content data in a memory card. If a device achieving the same security as the memory card is attached to the personal computer, the same distribution as that of the encrypted content data to the cellular phone can be performed for the personal computer.
Thereby, the personal computer receives the encrypted content data by installed software and the above device. Thus, the personal computer receives the encrypted content data of a different security level.
Further, music CDs storing music data are very popular, and ripping has been performed for obtaining the music data from the music CDs. The ripping produces encrypted music data (encrypted content data) from the music data and a license for decrypting and reproducing the encrypted music data. In the ripping, a watermark defining rules of use of the content data is detected from the content data, and the encrypted content data and the license are produced according to the contents of the watermark thus detected.
As described above, the cellular phone and the personal computer receive the encrypted content data and the license from the distribution server. The users of the cellular phone and the personal computer may intend to shift or copy the encrypted content data and the license thus received to a cellular phone or a personal computer of another user. In this case, the user can freely shift or copy the encrypted content data to the cellular phone or the personal computer of another user, but cannot freely shift the license, which is used for decrypting the encrypted content data, to the cellular phone or the personal computer of another user. When the license is shifted or copied to the cellular phone or the personal computer of another user, it is impossible to leave the license on both the sender side and receiver side in view of the copyright protection of the encrypted content data. Accordingly, the license on the sender side is deleted when the license is shifted or copied.
However, in a conventional license shift/copy method, when communication is interrupted during shifting or copying of a license to another user, this user on the receiver side cannot receive the license, and further the license is deleted on the sender side so that the encrypted content data cannot be decrypted with the license to be shifted or copied.